A String key for identifying the content. This value is specific to the service providing the content. If used, this should be a persistent unique key for the underlying content. It may be used with NoType:android/media/session/MediaController$TransportControls;Href=../../../reference/android/media/session/MediaController.TransportControls.html#playFromMediaId(java.lang.String, android.os.Bundle) to initiate playback when provided by a Android.Media.Browse.MediaBrowser connected to the same app.